A conventional optical modulator modulates optical intensity using Mach-Zehnder interference. In such an optical modulator, a Mach-Zehnder optical waveguide has a pair of parallel waveguides between an optical branch and an optical coupler. A two-input-two-output X-coupler may be used as the optical coupler. In this case, light output from the pair of parallel waveguides is coupled by the X-coupler and the resulting light is further split into two branches. One of the branches output from the X-coupler may be used for monitoring to adjust the phase of the light, which varying according to a bias voltage applied to the pair of parallel waveguides (see, e.g., Japanese Laid-Open Patent Publication Nos. 2005-274806 and 2001-7002).
Use of an X-coupler as an optical coupler of the Mach-Zehnder optical waveguide invites increases in the size of the chip on which the Mach-Zehnder optical waveguide is fabricated. When a two-input-one-output Y-coupler, which is shorter than the X-coupler is used as the optical coupler, the size of the chip can be reduced compared to that of the chip using the X-coupler. However, when the Y-coupler is used as the optical coupler and a portion of the modulated light (on-light) output from the Mach-Zehnder optical waveguide is used as monitor light, light (off-light) is emitted from the optical coupler. When a photodiode for monitoring receives the emitted light from the optical coupler, a problem arises in that the extinction ratio of the monitor light of the optical modulator drops.